1. Field of the Invention
The present invention is addressed to active matrix circuits for LC devices using fast, analog response liquid crystals with large molecular polarization in order to provide compact, high resolution reflective active matrix liquid crystal devices (AMLCD).
2. Discussion of Background
Circuits used to activate liquid crystal displays feature an active matrix circuit with one or two transistors supplemented by a storage capacitor in order to hold pixel voltage between consecutive frames. These circuits only function correctly for the liquid crystal when the liquid crystal polarization is sufficiently low or if the liquid crystal can be completely switched during one line address time T.sub.line, defined as ##EQU1## where .eta..sub.OH is an efficiency factor reflecting timing overhead, f.sub.R is the display refresh rate, N.sub.Row is the number of display rows, and N.sub.Color is the number of colors displayed time sequentially by a given pixel. As an example, a 60 Hz refresh rate, monochrome VGA resolution display has a line address time of approximately 32 microseconds, while a 75 Hz refresh rate SXGA display with three colors displaying time sequentially has a line address of approximately 4 microseconds.
If switching of the LC (liquid crystal) takes longer than one line address time, then the active matrix circuit must supply most of the charge needed to complete switching while at the same time maintaining the stored pixel voltage. Assuming negligible LC conductivity, the required charge is EQU Q.sub.Sw =2PA,
where P is the LC polarization in the switched state and A is the pixel area. In a worst case scenario, the LC switching time is much longer than the line address time. The factor 2 rises when the display is DC balanced by reversing pixel voltage polarity between consecutive frames.
The standard active matrix circuit including a transistor switch and a storage capacitor is not suitable for devices which use liquid crystals with large molecular polarization because, in order to supply Q.sub.Sw to the pixel, the storage capacitance C.sub.St must exceed the LC pixel capacitance C.sub.LC by approximately the number of resolvable gray scales or color shades, N.sub.GS wherein: EQU C.sub.St =N.sub.GS C.sub.LC.
This is required in order to prevent a reduction in the held pixel voltage by more than the equivalent of one gray scale. For 8 bit gray scale, a sufficiently large storage capacitance cannot be implemented using CMOS processes for fabricating active matrix back panels for reflective microdisplays.
In other types of devices which use twisted nematic liquid crystals (TNLC), the available storage node capacitance is not a limiting factor because the LC polarization is very small. Surface stabilized ferroelectric liquid crystal (SSFLC) devices are likewise unaffected by the limited available storage node capacitance because of their inherently bistable switching. The switched states of SSFLC can be stable even if the storage capacitor voltage drops appreciably while switching is being completed.
However, both types of liquid crystals have significant drawbacks to their use. Typical TNLC have switching times on the order of tens of milliseconds, while the bistable SSFLC can generate uniform and reproducible gray scale only by temporal or spatial multiplexing (sub-pixels). These techniques in turn either raise the response time or increase the pixel area, by a factor equal to the number of resolved gray scales.
Of considerable interest in this area is the development of Electroclinic Liquid Crystals (ELC), also known as Soft Mode FLC, which combines the gray scale response of TNLC with the fast switching of SSFLC. Recently ELC have been synthesized featuring fast switching of approximately 100 microseconds and a gray scale response at low applied fields of a few volts per micrometer. Further they have a high contrast of greater than 500 to 1 and a large depth of modulation because of high tilt angles of up to 30 degrees. Of significance however is the fact that the induced polarizations can reach several hundred nC/cm.sup.2 which cannot be operated properly using prior art active matrix cell circuits. Accordingly there is a need for a differently constructed matrix cell circuit to handle the larger induced polarizations.